Many of the oncogenes implicated in the origins of human cancers are thought to be involved in pathways that link extracellular signals to the machinery that governs the cell cycle. Thus, when these signals are altered by mutations, cellular proliferation becomes aberrantly regulated. Despite the importance of this subject, we still have an incomplete picture of how extracellular signals regulate the mitotic cycle. We have been using Saccharomyces cerevisiae as a model system to study how extracellular nutrient signals regulate proliferation. We have found two different pathways that allow nutrients to regulate the activity of the conserved cyclin/p34CDK pathway that governs the cell cycle. One pathway involved homologues to the p21ras oncogene products, which in budding yeast stimulate cyclase, and production of cAMP. The other pathway is cAMP-independent. Cyclic AMP induces the transcription of tow G1 cyclin genes, CLN1 and CLN2, while the cAMP-independent pathway induces the transcription of another G1 cyclin gene, CLN3. This proposal is aimed at determining the mechanism by which nutrients produce these responses in yeast cells. Specifically we want to know how cells increase CLN1 and CLN2 transcription in response to cAMP, and how nutrients produce the cAMP-independent increase in CLN3 transcription. Answers to these questions will help us to understand how cells regulate proliferation in responses to extracellular signals. In order to address these questions we propose the following specific aims: 1. We observe an increase in p34 CDC28 activity in response to cAMP that is sufficient to explain the increase in CLN1 and CLN2 transcription. We will determine the mechanism by which cAMP increases the activity of p34CDC28. 2. We will map 5' regulatory regions of the CLN3 gene to identify elements that confer increased transcription in response to nutrients. 3. We will isolate mutants that fail to increase G1 cyclin transcription in response to cAMP and nutrients. These mutants will help us identify genes that are important in regulating G1 cyclin expression in response to nutrients.